Update: cat's right eye is also swelling shut
From what you describe, it really does sound like your cat got into a fight or possibly got hit by a car(God, I hope not). I have a lot of experience with cats-30+ yrs!
The eye swelling along with the lump on the head makes me very nervous and I would take your cat to the vet ASAP. They will actually shave the area on his head to see if there is scratch, bite marks or any other injury. If it's an abscess(that's what I'm leaning towards), they will drain it and put your cat on antibiotics.
Good luck. Please come back with an update!